This invention relates to the field of absorption spectroscopy and, in particular, to a cavity-ringdown system for the determination of ringdown rates by optical heterodyne detection.
Traditional spectroscopic methods are limited in sensitivity to approximately one part per ten thousand (1:104) to one part per hundred thousand (1:105). The sensitivity limitation arises from instabilities in light-source intensity translated into noise in the absorption signal.
The use of optical resonators for enhancing absorption contrast is described by Kastler (xe2x80x9cAtomes à l""Intxc3xa9rieur d""un Interfxc3xa9romxc3xa8tre Perot-Fabry,xe2x80x9d Appl. Opt. 1, 1 (1962) pp 17-24) and implemented by Cerez et, al. (xe2x80x9cHe-Ne Lasers Stabilized by Saturated Absorption in Iodine at 612 nm,xe2x80x9d IEEE Trans. Instrum. and Meas. 29, 4 (1980) pp 352-354) and Ma et. al. (xe2x80x9cOptical Heterodyne Spectroscopy Enhanced by an External Optical Cavity: Toward Improved Working Standards,xe2x80x9d IEEE J. Quan. Electron. 26, 11 (1990) pp 2006-2012). Cavity Ring-Down Spectroscopy (CRDS), first described by O""Keefe and Deacon in xe2x80x9cCavity ring-down optical spectrometer for absorption measurements using pulsed laser sources,xe2x80x9d in Rev. Sci. Instrum. 59, 12 (1988): pp 2544-2551, allows absorption sensitivities of 1xc3x9710xe2x88x927. The applications of CRDS include measurement of ultra-slow reflector velocities, atmospheric sensing, detection of trace species in gas-phase environments, absolute determination of absorption-band strength and/or species concentration, analysis of combustion and plasma dynamics, study of chemical kinetics (such as radical reactions and internal vibration redistribution), and characterization of optical cavities and high-reflectivity mirror coatings. Recently, CRDS has been applied to surface and condensed matter (Pipino, xe2x80x9cUltrasensitive Surface Spectroscopy with a Miniature Optical Resonator,xe2x80x9d Phys. Rev. Lett. 83, 15 (11 Oct. 1999) pp 3093-3096), thus permitting a wide range of novel fundamental investigations.
In a CRDS system, a sample (absorbing material) is placed in a high-finesse stable optical resonator or ringdown cavity. The light completes many roundtrips through the intra-cavity absorber, effectively increasing the interaction length by 2xc2x7Finesse/xcfx80. Light admitted into the ringdown cavity circulates back and forth multiple times setting up standing waves having periodic spatial variations. Light exiting the ringdown cavity is proportional to the intra-cavity light intensity.
The radiant energy stored in the ringdown cavity decreases in time (rings down). For an empty cavity, the stored energy follows an exponential decay characterized by a ringdown rate that depends only on the reflectivity of the mirrors, the separation between the mirrors and the speed of light in the cavity. If a sample is placed in the resonator, the ringdown is accelerated. Information about intra-cavity gas absorption is obtained by measuring the change of decay associated with the cavity field. An unknown absorption coefficient is compared to known mirror losses. The mirror losses may have a magnitude similar to the unknown absorption coefficient in order to reduce background detection, thus enhancing contrast between the unknown absorption coefficient and the background (i.e., mirror losses). An absorption spectrum for the sample is obtained by plotting the reciprocal of the ringdown time T or the decay constant 1/xcfx84 versus the wavelength xcex of the incident light.
U.S. Pat. No. 5,528,040 describes a CRDS system in which the decay rate of the ringdown cavity cell is calculated from a signal produced by a photodetector that is responsive to radiation resonated by the cell. The calculated decay rate is used to determine the level of trace species in the sample gas. The method measures cavity ringdown using a continuous wave laser. The cavity transmitted power is used to monitor the intracavity absorption. Lacking an efficient differential comparison mechanism, intensity noise of the diode laser used in the method places substantial limit on the achievable absorption sensitivity.
U.S. Pat. Nos. 5,986,768 and 5,835,231 describe elegant setups of high finesse optical resonators that permit measurement of absorption using evanescent waves to provide spatial resolution. However, the technique employed is the commonly used single beam cavity ringdown.
In CRDS, a pulsed operation produces an abrupt termination of the cavity input field, which permits a measurement of the exponential decay curve of the cavity-transmitted power. Intensity fluctuations of the incident light are not related to the ringdown rate in the ringdown cavity and thus, they do not directly affect the CRDS measurement. Thus, this cavity ringdown method avoids the noise in the light source. However, residual fluctuations in the apparent cavity loss prevent this method from achieving the performance suggested by fundamental noise limits. For example, if CRDS were only limited by shot-noise inherent in any light beam due to the quantum nature of the photons constituting the light beam, the achievable sensitivity would be in the range of 10xe2x88x9214 cmxe2x88x921 Hzxe2x88x92xc2xd.
Various improvements to CRDS are well known. For example, U.S. Pat. No. 5,528,040 describes a laser-diode source for CRDS. The diode laser is optically locked using controlled optical feedback from a reference cavity to improve the coupling of light into the ringdown cavity. U.S. Pat. No. 6,084,682 describes a CRDS system that uses separate sampling and locking( light beams. The sampling and locking beams are provided with different wavelengths. U.S. Pat. No. 5,912,740 describes a ring resonant cavity that eliminates feedback into the light source. The absence of feedback to the light source leads to reduced frequency fluctuations, improved light-cavity coupling, reduced baseline noise, and increased absolute sensitivity. U.S. Pat. No. 5,815,277 describes an acousto-optic modulator used to couple light into a CRDS resonant cavity. U.S. Pat. Nos. 6,097,555 and 5,973,864 describe utilizing Brewster""s angle prism retro-reflectors.
There are two basic limitations to conventional CRDS. One of these limitations is due to the DC nature of CRDS. For example, two decay-ti me measurements are made, one for an empty cavity and the other for a cavity containing a sample. The difference between the two measurements contains useful information. However, when there is a large time difference between the two measurements, slow drift and various noise factors contaminate the data.
Another limitation of CRDS is the requirement that a CRDS detector have a large dynamic range to record data. Typically, a lower portion of the exponential CRDS decay curve is masked by instrument noise because insufficient power is available for the decay curve to be distinguishable from electronic noise.
Accurate measurements of small signal changes with a varying background signal can be achieved with a precise signal-extraction method and averaging. Modulation techniques are typically employed to distinguish decay-time measurements from background signals so that any drifts and noise in the background can be removed. In xe2x80x9cUltrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy,xe2x80x9d J. Opt. Soc. Am. B 15, 1, pp 6-15 (1998), which is hereby incorporated by reference, a frequency-modulation technique enables shot noise limited absorption sensitivity in sub-Doppler resolution. On-resonance and off-resonance information are compared at a radio-frequency (RF) rate, which is located away from the laser-intensity noise spectrum.
Ye et al. (xe2x80x9cUltrasensitive Detection in Atomic and Molecular Physics: Demonstration in Molecular Overtone Spectroscopyxe2x80x9d, Journal of the Optical Society of America B, 15, 1, (January 1998), pp. 6-15) teaches a heterodyne technique building on spectroscopic techniques employing frequency modulation (FM) detection.
Levenson et. al. (xe2x80x9cOptical heterodyne detection in cavity ring-down spectroscopy,xe2x80x9d Chem. Phys. Lett. 290 (1998) pp 335-340) describes a heterodyne technique used to superimpose a large local-oscillator field onto a decay field so that the resultant beat frequency is only light-noise limited.
U.S. Pat. No. 6.094,267 describes an optical heterodyne detection technique that improves the detection sensitivity of a CRDS system such that the sensitivity approaches the shot-noise limit. A local-oscillator signal and a signal wave are coupled into a ringdown cavity containing a sample. The local-oscillator signal and the signal wave have different frequencies. To perform the ring-down measurement, the signal wave is interrupted, such as by chopping the wave or changing the signal frequency. An exponentially decaying ringdown signal output from the cavity is combined with the uninterrupted local-oscillator signal to produce a heterodyne beat frequency. However, this technique does not offer the possibility of a quick comparison of on-resonance and off-resonance information, which is key to achieve Quantum noise limited sensitivity. Also, this technique still requires recording the entire ringdown decay curve, which needs a substantial dynamic range to record accurately the decay curve.
Unfortunately, the above adaptations are not well designed to measure exponentially decaying waveforms. In particular, these techniques do not work well for signal detection in CRDS. It is desirable to employ a single technique that simultaneously addresses the two basic limitations of CRDS. It is also desirable to provide a method that approaches the fundamental quantum-noise limit in cavity-enhanced linear spectroscopy.
An object of the present invention is to provide a CRDS system that is substantially immune to slow drift and various noise factors that typically contaminate data, and therefore enable a true quantum noise limited is detection sensitivity.
Another object of the invention is to provide a CRDS system that reduces the dynamic range of a CRDS detector required to accurately record data.
The present invention compares two slightly different time constants. One time constant is associated with an empty cavity. The other time constant is associated with cavity loss plus an additional loss. Using cavity filtering and an intensity-stabilized laser, it is possible to approach (within a factor of four) the fundamental quantum-noise limit in cavity-enhanced linear spectroscopy.
The invention uses heterodyne detection of two modes having a relative frequency offset that are chopped out of phase and coupled into a resonating cavity. One of the modes may be tuned to an absorption line of a sample gas in the cavity. In a first half-cycle, a heterodyne-beat signal results from one mode that is coupled into the cavity (and thus, rising exponentially in intensity) beating against a second mode that has been switched off (and thus, is diminishing exponentially in intensity). In a second half-cycle, the first mode is switched off and the second mode is switched on. Absorption of one of the modes by the sample gas is easily detected by observing the difference between adjacent half-cycles of the heterodyne beat signal.